Microstructure stiction frequently occurs in micro-electro mechanical (MEM) devices due to wet chemical processing of the microstructure. Typically, stiction occurs when surface adhesion forces are higher than the mechanical restoring force of the microstructure. For example, stiction may occur after wet etching of an underlying sacrificial layer, when a liquid meniscus formed on hydrophilic surfaces of the microstructure pull the microstructure toward an associated substrate. Usually, such stiction problems may be alleviated by a dry hydrofluoric (HF) etching or supercritical carbon dioxide (CO2) drying.
A more difficult problem to address is in-use stiction, which occurs during operation when components of the microstructure come into contact, intentionally or accidentally. In-use stiction may be caused by capillary forces, electrostatic attraction and/or direct chemical bonding. Developers have used alternative process, such a bulk micromachining, to decrease the chance of in-use stiction. However, these processes are generally less capable and versatile than surface micromachining, in terms of device function. However, even bulk micromachined MEM devices may still experience in-use stiction.
One approach to address the in-use stiction problem associated with MEMs structures has been to provide a low-energy surface coating, in the form of an organic passivation layer, on the inorganic surfaces of the microstructures. In general, such coatings can eliminate or reduce capillary forces and direct chemical bonding, as well as reduce electrostatic forces. For example, Texas Instruments has utilized a fluorinated fatty acid self-assembled monolayer (SAM) on an aluminum oxide surface in their digital micro-mirror device (DMD). As another example, Analog Devices has coated the surface of their inertia sensors using thermal evaporation of silicone polymeric materials at the packaging stage, after the device is completely released. Another approach has utilized the formation of siloxan SAMs on the oxide terminated surface of the microstructures. However, the use of siloxan SAMs is difficult to implement because of the chemistry and, as such, its poor reproducibility limits its practical usage. Furthermore, experimental evidence has shown that wear-resistance films, such as SAM coatings, are removed during the operation of the MEM device. In addition, many organic layers have a limited lifetime or decrease the allowable operating temperature of the device.
Another technique for addressing in-use stiction in MEM devices has been to form stiction bumps in a polysilicon surface. In this technique, the stiction bumps are formed in the polysilicon surface of the micromachined structure by patterning the surface of a sacrificial layer. Thus, when the polysilicon is deposited, it takes on the shape of the patterned layer forming a bump, when the sacrificial layer is removed. However, this technique cannot be utilized for silicon-on-insulator (SOI) wafer based MEM devices, as the sacrificial layer is part of the SOI wafer.
What is needed is a technique for forming bumps on a surface of a movable micro-electro mechanical structure that is fabricated in an assembly, e.g., a silicon-on-insulator wafer, having an internal sacrificial layer.